The vivid interest in organic semiconductor materials is stimulated by the technological aspects of applications that employ these new materials in organic light emitting diodes (OLEDs), thin-film field-effect transistors, solar cells and biological sensors. Some products, such as emissive OLED displays, have gained rapid acceptance in portable displays because of their bright emission and low power consumption. The remarkable technological breakthroughs in the development of highly efficient OLEDs and the demonstration of optically-pumped lasing using the same class of organic semiconductors have focused an increasing interest in the prospects for making electrically-pumped organic solid-state lasers. Lasing action has been studied in a variety of optically pumped structures (e.g., microcavities, distributed feedback (DFB) structures, . . . ) that demonstrate the feasibility of organic thin-film materials as active laser media.
Electrical pumping of organic light-emitting devices, however, has to date not been proven successful in achieving any lasing emission. The combination of several issues makes electrical excitation a particularly tough problem for organic semiconductor lasers. They suffer from considerable losses originating from the electrical contacts, the depletion of excitations by junction current (exciton-polaron quenching), absorption by polarons, exciton-exciton annihilation, etc. . . . One of the main problems associated with electrically pumped organic semiconductor lasers is the inevitable population and accumulation of triplet excitations, which results in excessive triplet-state losses preventing lasing (i.e. which results in excessive losses caused by triplet-state excitations, and which in turn will prevent lasing), as e.g. described by S. Verlaak et al. in “Numerical simulation of tetracene light-emitting transistors: a detailed balance of exciton processes”, Applied Physics Letters, 2004, Vol. 85, p 2405. If conventional spin statistics applies, the recombination of injected charge carriers leads to the creation of a majority (e.g. about 75%) of (non-emissive) triplet excitations in the active organic semiconductor layer. Due to their long lifetime, these triplet excitations can act as metastable species, which generally have fairly high absorptions to the upper triplet state (triplet-triplet absorption) at the expected fluorescent lasing wavelength. Such a problem is inherent to organic materials and is a well-known issue also in classical liquid-state organic dye lasers employing highly fluorescent organic dye molecules dissolved in organic solvents. Triplet-state losses (i.e. losses caused by triplet-states) often limit the lasing performance of these dye lasers, especially at continuous wave (cw) operation. Consequently, organic dye lasers normally operate in pulsed mode (with a pulse duration of several nanoseconds) using a short-pulse flash-lamp for pumping, since only in this way the accumulation of dye molecules in the triplet state can be overcome.
To circumvent this problem, a commonly accepted practice in liquid-state organic dye lasers is to use the so-called triplet (excitation) scavengers. It was demonstrated that the accumulation of dye molecules in the triplet state can be reduced by adding such triplet scavenging molecules to the dye solution (e.g. as reported by J. B. Marling et al in “Chemical quenching of the triplet state in flashlamp-excited liquid organic lasers”, Applied Physics Letters, Vol. 17, Nr. 12, 1970). In this way quenching of lasing emission by triplets might be almost eliminated. To reduce triplet accumulation, the triplet (excitation) scavenger molecule should meet a set of important requirements. It should have the ability to accept a triplet excitation from the dye molecules, implying that its triplet level should be sufficiently low. At the same time its singlet level should be high enough to prevent quenching of singlet excitations of the dye molecules. This implies thus that the S1-T1 splitting for the triplet scavenger should be extraordinary large. Furthermore, it should posses a reasonably short intrinsic triplet lifetime to deplete quickly the triplet population and/or have intrinsic triplet-triplet absorption shifted far from the region of lasing of the dye molecules. In addition it should not enhance intersystem crossing of the dye molecules to prevent conversion of singlets into triplet excitations. This criterion imposes limitations for employing compounds containing heavy atoms (such as e.g. metal-organic complexes) as triplet scavengers. Taking into account all these requirements, it is not surprising that there are just a very limited number of efficient triplet scavengers available today.
A cyclic nonaromatic polyene, 1,3,5,7-cyclooctatetraene (COT) is the most popular and efficient triplet scavenger used in liquid-state organic dye lasers. This is related to its unique combination of properties, namely the ability to quench host triplets with an energy as small as 0.8 eV and its very short triplet lifetime (˜100 μs). The use of COT as an efficient triplet scavenger for several laser dye solutions is well documented. The mechanism for the energy transfer process has also been investigated in detail. Unlike molecular oxygen, which is known as a notorious triplet quencher due to its triplet ground state, COT can quench the triplet state without increasing the intersystem crossing rate and without oxidation of the dye molecules. To the best of the Applicant's knowledge, COT was used so far only in dye solutions. COT is a flexible molecule in the ground state and belongs to the “non-classical” triplet acceptors, which exhibit anomalous non-vertical (non-adiabatic) triplet energy transfer. It is known that quenching of a host triplet by a non-vertical triplet scavenger such as COT requires a planarization (and thus a structural reorganization or geometrical distortion) of the COT molecule during the triplet transfer. Given the need for a structural reorganization, it can be expected that triplet quenching by non-vertical triplet energy transfer can only be obtained in an environment wherein such a structural reorganization is not hindered, such as e.g. in a liquid environment.
Classical vertical triplet energy transfer has been well documented for solid films of conjugated semiconducting polymers such as PPV derivatives, polyfluorenenes and MeLPPP. However, it is very difficult to find vertical triplet scavengers that meet the requirements (described above) for reducing triplet accumulation. An important requirement for a suitable triplet scavenger is a very large S1-T1 splitting. In addition the triplet scavenger should (preferably) not enhance intersystem crossing of the host molecules. An example of a vertical triplet acceptor meeting these requirements is anthracene. However, this molecule has a long triplet lifetime (several tens of ms), such that the triplet population is not reduced.
Aims of the Invention
It is an aim of the present invention to provide methods for efficient triplet scavenging or quenching in solid state organic materials, e.g. in solid state organic light-emitting materials, in order to reduce a total triplet population. This aim is achieved by incorporating a molecule exhibiting non-vertical triplet energy transfer (such as e.g. a non-vertical triplet scavenger) in the solid state organic material or at a distance from the solid state organic material that is smaller than the triplet exciton diffusion length, e.g. at a distance smaller than 100 nm.
It was surprisingly found that this allows obtaining an efficient reduction of the triplet population in solid state organic materials (without reducing the singlet exciton density. This effect is due (preferably) to a large splitting between a singlet level and a (relaxed) triplet level of the scavenger, a short intrinsic triplet lifetime of the scavenger, and the absence of any enhancement of intersystem crossing of the solid state organic molecules.